1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a digital multi-functional system including a combination thereof, and more particularly, to an image forming apparatus that transfers a toner image formed on a photoreceptor to a transfer medium such as a recording medium through an intermediate transfer member.
2. Description of the Background Art
Consistent high-quality imaging for an extended period of time is desired of image forming apparatuses such as copiers, facsimile machines, printers, and so forth. However, in the image forming apparatuses, the toner used to form images is degraded with time, which adversely affects imaging quality.
Typically, in the image forming apparatus, which includes a developing device, an unfixed toner image is developed with a developing agent, for example, a two-component developing agent (hereinafter referred to as a developing agent) consisting of a charged toner and a carrier to form a visible image also known as a toner image. The developing agent is borne by developing agent bearing member, and in order to optimize the amount of the developing agent on the developing agent bearing member, a developing agent regulator or the like is provided to the developing device.
Unfortunately, the toner is subjected to repeated mechanical stress by the developing agent regulator, the practical effect of which is to degrade the toner by causing the charge on the toner to fluctuate undesirably. This complicates efforts to achieve a desirable image density, and also causes contamination of an interior of the device and undesirable toner adherence to recording media sheets.
To address such difficulties, there is known a method for replacing degraded toner with fresh toner when the ratio of degraded toner to total amount of toner reaches a certain level. For example, Japanese Unexamined Patent Application No. 2006-171788 (JP-2006-171788-A) proposes to consume forcibly the toner outside an imaging region when an image area is small and thus a small amount of toner would otherwise be consumed. Degradation of toner is significant with a small amount of toner consumption because the toner is repeatedly stressed. For this reason, the toner is consumed forcibly outside the imaging region to replace the toner with fresh toner.
Disadvantageously, however, the toner is wasted when the toner is consumed forcibly in this approach, thereby increasing operating costs. Moreover, in this approach, when a small amount of toner is anticipated to be consumed, the toner is forcibly replaced with fresh toner. Therefore, the real amount of degraded toner is difficult to determine.
Even when the developing agent contains degraded toner, good imaging quality is still desired of the image forming apparatus. In another approach for achieving high imaging quality for an extended period of time, a process control is performed by measuring an amount of toner adhered to a toner test pattern formed on an image bearing member, for example, a photoreceptor. In this approach, a toner pattern is formed on the photoreceptor and then transferred onto an intermediate transfer member. The amount of toner adhered to the toner pattern is detected and taken into account in setting toner image forming conditions, such as a charging condition, a developing condition, and so forth.
This approach is advantageous because the actual amount of toner adhered to the toner pattern is measured and the toner image forming condition is determined based on the actual amount of the adhered toner, making it relatively easy to obtain a proper amount of toner to adhere to a toner image.
However, there is a drawback to this configuration in that degraded toner tends to be difficult to transfer properly. Therefore, even if the proper amount of toner is adhered on the photoreceptor, if the toner image on the photoreceptor contains degraded toner, the amount of toner adhered to the toner image transferred onto the recording medium becomes uneven, thereby reducing imaging quality.
Furthermore, the present inventor has found that a certain relation exists between the degraded toner and a toner transfer efficiency in an image forming apparatus that transfers a toner image formed on a photoreceptor onto a recording medium through an intermediate transfer medium.
Typically, primary transfer conditions for primary image transfer from the photoreceptor to the intermediate transfer medium are optimized such that the primary transfer is performed with a greater tolerance, so that the degraded toner in the toner image on the photoreceptor can still be transferred. In such a primary transfer process, a decrease in a primary transfer efficiency with respect to the intermediate transfer member is not significant, and thus the amount of toner adhered to the toner image is maintained relatively even.
By contrast, in a secondary transfer process in which the toner image is transferred from the intermediate transfer member onto the recording medium, the degree of tolerance is set to be generally low, and the secondary transfer condition is set to achieve a good transfer efficiency with the toner that is not degraded. In such a secondary transfer condition, if the toner image on the intermediate transfer member contains degraded toner, the degraded toner is difficult to transfer, thereby causing undesirable reduction in the secondary transfer efficiency and unevenness in the amount of toner adhered to the toner image formed on the recording medium.
Referring to FIG. 1, there is provided a graph showing a comparison of adherence of toner using a developing agent with fresh toner and using a developing agent with degraded toner.
In FIG. 1, granularity refers to a characteristic value representing uniformity in the amount of toner adhered to the toner image. The good evenness makes the granularity small.
As illustrated in FIG. 1, when the developing agent containing degraded toner is used, the difference in the granularity in the new developing agent containing only an initial toner and the developing agent containing degraded toner is insignificant on the photoreceptor and on the intermediate transfer member. However, the granularity increases when the toner image is transferred onto the recording medium before the toner image is fixed. In other words, after the secondary transfer process, the amount of toner adhered to the toner image becomes uneven when the toner includes degraded toner. As a result, the image quality on the recording medium is undesirably reduced.
In view of the above, the secondary transfer conditions may be set such that a good transfer efficiency is still achieved even when the toner contains degraded toner. In one approach, for example, a relatively large electric current is set for the secondary transfer as the secondary transfer condition, thereby facilitating transfer of degraded toner. However, a large electric current may adversely affect the transfer efficiency relative to normal toner, that is, toner that is not degraded.
Another approach includes increasing a secondary nip pressure as the secondary transfer condition. This approach has also a drawback in that increasing the nip pressure increases mechanical stress, thus causing fluctuation of the speed of sheet transportation and degradation of sheet transportability, again adversely affecting imaging quality.
In view of the above, there is demand for a device that prevents degradation of secondary transfer efficiency caused by degraded toner and provides consistently high imaging quality for an extended period of time.